1. Field of the Invention
The present invention relates to a noise absorber which prevents electromagnetic noise from being radiated from or entering a signal line between electronic devices, for example, between a computer and its terminal devices.
2. Description of the Related Art
Electromagnetic signals transmitted through a signal line can radiate from the signal line and enter into another signal line. Electromagnetic signals that enter from one signal line into another signal line are usually harmful because such electromagnetic signals cause electromagnetic noise which interferes with the original signals intended to be transmitted through the other signal line.
Conventionally, many type of electromagnetic noise absorbers have been proposed. As shown in FIG. 8, a conventional electromagnetic noise absorber 1 comprises a cylindrical ferrite in which a through hole 2 is provided. The electromagnetic absorber 1 is attached to a signal line 3 with the signal line 3 inserted into the through hole 2 of the electromagnetic noise absorber 1. The signal line 3 can be inserted in the through hole 2 twice so as to form a loop 3a, as shown in FIG. 9. This increases the impedance of the signal line 3, thereby reducing electromagnetic noise.
The electromagnetic noise absorber 1, however, has the following drawbacks. Specifically, it is impossible to insert a signal line into the through hole of the electromagnetic absorber 1 if the signal line has connectors larger than the diameter of the through hole 2. The electromagnetic noise absorber 1 is also inconvenient if the signal line 3 is considerably long. Moreover, if the loop 3a is formed, as shown in FIG. 9, the signal line 3 may be easily damaged at the bent portions of the loop 3a.
In order to provide an electromagnetic noise absorber which can be attached to a signal line having terminal connectors, there has been proposed an electromagnetic noise absorber 5 shown in FIG. 10. The electromagnetic noise absorber 5 comprises a pair of hemi-cylindrical ferrites 6 and 7 and a case 8. The hemi-cylindrical ferrites 6 and 7 are formed by cutting the cylindrical ferrite 1 of FIG. 8 into two pieces along the central axis so that the signal line 3 can easily be placed in the through hole 2 of the cylindrical ferrite 1. The case 8 consists of a pair of containers which are made of an insulating material and are connected to each other at the bending portion 8a. Each of the containers holds one of the hemi-cylindrical ferrites 6, respectively. A protrusion 8b and a hook 8c are provided for locking the containers of the case 8.
When the electromagnetic noise absorber 5 is attached to a signal line (not shown in FIG. 10), the signal line is placed in one of two divided grooves 2a and 2b which together constitute the through hole 2 and the case 8 is folded such that the hemi-cylindrical ferrites 6 and 7 form a cylindrical shape. The protrusion 8b is then fixed to the hook 8c, whereby the signal line is held in the through hole 2.
Although the electromagnetic noise absorber 5 may solve the problem associated with attaching the electromagnetic absorber 5 to a signal line with terminal connectors, the electromagnetic noise absorber 5 has a different problem in that it is difficult to provide the signal line with a high impedance.
For the foregoing reasons, there is a need for an electromagnetic noise absorber which is easily attached to a signal line and provides a signal line with a high impedance.